From Federal Republic of Germany 32 06 981 C2, a method for the compression molding of molded parts from, for instance, sintered metal powders is known in which the molded parts are formed of different material in separate regions (composite moldings). In that method, the mold cavity formed by the matrix and the bottom plug, when in a first filling position, of a compression mold is first filled flush with the surface by a first powder material. The mold cavity is then enlarged further by lowering the bottom plug into a second filling position, whereupon it is filled again, flush with the surface, by a second powdered material. Since the lower part of the matrix has a step which protrudes into the inside of the mold and with respect to which the bottom plug is lowered, the enlargement of the mold cavity is developed in the shape of a trough in accordance with the slope curve of the first powder.
Therefore, the line of separation between the two types of powders can always only be in the form of this slope curve. This means that vertical lines of separation between the materials cannot be obtained at all by this method. Molded parts in the shape, for instance, of gear wheels in which the actual wheel body is to consist of a simple structural steel and only the comparatively narrow zone of the toothing region is to be formed of a high-alloy wear-resistant steel cannot be produced with it.
This limitation is no longer present in the case of the method of manufacturing composite moldings from two different alloy powders known from Federal Republic of Germany 33 05 879 A1. That result is achieved in the manner that a preform of the first alloy powder is inserted into the compression mold, the remaining base of the mold is filled with the other alloy powder, and the two powders are finally compressed together to form the desired molded part. The preform must, for this purpose, have a dimensional stability which is sufficient for handling. For this reason, a resin binder is first of all added to the alloy powder in question. The powder is then formed in a core shooter, which is known from foundry technology, with mechanical compression, into the preform. These preparations therefore are carried out outside of the mold in which the actual composite molded body is produced. This results not only in a corresponding handling expense for bringing the different materials together but also in the providing of a core shooter. Furthermore, there is the disadvantage that the resin binder must be burned out from the compacted molding before or during the final sintering.
From U.S. Pat. No. 4,353,155 it is known to manufacture a molded body formed of two layers of different powdered metal materials, namely a bronze layer and an iron layer, for use as plain bearing. In this case, the iron powder is first of all introduced into a cylindrical mold cavity, formed by a matrix into which a center pin has been coaxially introduced. The center pin which is developed with steps and is of small diameter at its upper part is pulled out after the filling, displacing part of the iron powder which has already been introduced. This displaced part must, first of all, be removed before a pre-compacting of the iron powder in the matrix is effected by compressing the powder charge between an upper plug and a lower plug. The upper plug has a central bore into which the upper part of the center pin is slidingly introduced. During the compression process, the pressing surface of the upper plug is always flush with the upper end surface of the matrix and therefore does not dip into the mold cavity of the matrix. The latter takes place only in the case of the lower plug. When a sufficient green strength has been reached, and therefore the pre-compact produced no longer falls apart upon the removal of the center pin, the center pin is moved downward to such an extent that its upper part of smaller diameter lies coaxially within the pre-compact. In this way there is produced an open mold space between the pre-compact and the center pin, which cavity is filled with the bronze powder. Thereupon, the final compressing of the compact is effected by the action of upper and lower plugs on the entire mass of powder, the compact being sintered in known manner after removal from the mold.
This method has considerable disadvantages. First of all, the introduction from one side of only the lower plug into the mold cavity upon the production of the pre-compact leads to a pre-compacting which is nonuniform in the direction of compression as a result of the friction between the column of powder and the walls of the matrix. Furthermore, the handling of the powder is very cumbersome since the remaining quantity of the powder which has been pressed out of the matrix, which quantity differs depending on the filling factor, must be removed. For this purpose, the quantity of powder can be blown away, for instance, by compressed air or be swept away by hand with a broom.
Both increase the cost of manufacture of the compacts. If it were desired simply to push the excess powder away after the effecting of the pre-compacting, this would be possible only with the center pin withdrawn, and therefore with the mold space for the bronze powder open. Accordingly, a part of the iron powder would drop into this mold space and lead to "material contamination" there. Furthermore, there is the disadvantage that the relative compacting in the two powder layers must necessarily come out differently upon the joint final compacting since the height of the mold space for the bronze powder is less than that for the iron powder, namely corresponds precisely to the height of the pre-compact and, accordingly, the pressing paths are also different. In the manufacture of plain bearings, this is permissible or even desired since an open pore structure of the bronze layer can favor the lubricating properties. For the manufacture of parts of the highest possible density and, in particular, approximately the same relative compression in the individual layers of material, this method is unsuitable. Nor is this method suitable for purposefully establishing desired degrees of compaction in the individual layers of material since the height of the mold space for the bronze powder can never be greater than the height of the pre-compact which is to be established for the minimum green strength. Furthermore, this method does not permit the production of composite layer bodies having more than two layers of material.
It is therefore the object of the present invention to improve a method of this type in such a manner that molded parts having vertically extending separation lines between the individual portions of the composite material can be produced without the disadvantages described, and therefore with any relative compression and, in particular, with practically the same relative compression. Furthermore, there is to be created an apparatus on which the compacts can be produced in accordance with this method.